The Ottoman Turks were originally based in western Anatolia and had risen to prominence as a frontier principality on the eastern borders of the Byzantine Empire during the thirteenth and fourteenth century. By the mid-fifteenth century, the Ottoman sultanate had conquered much of Anatolia, Greece, Thrace, and the Slavic-speaking regions south of the Danube; in effect, they had replaced the Byzantine Empire as the dominant power in the Balkans and the Aegean. The culmination of Ottoman expansion in southeastern Europe was the conquest of Constantinople on May 29th 1453, which was accomplished after a fifty-four-day siege by Sultan Mehmed II (r.1451–1481), known as “the Conqueror” following his capture of the Byzantine capital.
By the late 1230s, Mongol armies had begun raiding parts of Russia and eastern Europe. Between 1236 and 1242, these military campaigns–commanded by Subutai (d. 1248), Batu Khan (d. 1255), and Berke (d. 1266), among others–had wrought major devastation across Russia, Poland, Hungary and the Balkans. The cities of Kiev, Pereyaslavl, Chernihiv, Lublin, and other major population centers in eastern and central Europe were sacked and their populations massacred. The defeats of the Polish forces at the Battle of Liegnitz/Legnica (April 9th 1241) and the Hungarian military at the Battle of Mohi (April 11th 1241) opened up most of the Balkans and Central Europe to Mongol raids, leading to even more destruction, displacement and massacres. These alarming developments shook the foundations of Latin Christendom. Although the Mongols withdrew from most of the Balkans and east-central Europe soon after (as a result of internal dynamics in their empire), the shock of their invasions and conquests remained.
Seeking to gauge the intentions of the conquerors and convince them to cease their invasions of Latin Christendom, Pope Innocent IV (r. 1243-1254) sent an embassy with two letters to the Mongol Khan Güyük. The leading papal legate to the Mongols was the Italian Franciscan Giovanni da Pian del Carpine (d. 1252) who departed Lyon in April 1245 & arrived at the court of Güyük Khan near Karakorum in July 1246. He returned to Lyon by the end of 1247. Carpine composed the Ystoria Mongalorum, a detailed account of the mission & his extensive travels (more here). The Latin text & English translation of Carpine’s travel account and history of the Mongols can be found here (along with the writings of William of Rubruck).
(Pope Innocent IV sends Dominicans and Franciscans out to the Mongols. Vincent of Beauvais, Le Miroir Historial Vol. IV, Paris, c. 1400-1410.